The traditional beamhouse processes or wet processing cleans the hides or skins and prepare them for further processing like retanning, fat liquoring, dyeing and finishing. The beamhouse process includes the steps of soaking (dirt removal and re-hydration), dehairing (removal of hair, traditionally part of the liming process), liming (removal of hair and release of fats and proteins as well as swelling of the collagen structure), fleshing (removal of fatty tissue), splitting (horizontal cutting into grain split and flesh split), deliming (releasing lime and reducing pH), bating (removal of proteins, scut removal and fiber opening), and pickling (lowering of pH value to around 3) and tanning (stabilization of the skin or hide matrix). The product of this process is generally known as wet-blue.
Enzymes have been used in the leather industry for around 100 years (Uhlig, 1998, in Industrial Enzymes and Their Application, chapter 5.9 by John Weiley & Sons). At present, enzymes are used with relative success in soaking, dehairing, bating and degreasing (Thanikaivelan et al., 2004, Trends in Biotechnology 22:181-187).
Proper removal of hair from the outer surface and the hair follicles is very important to ensure soft and smooth surface of the grain and to ensure evenness in color of the leather. The most commonly practiced method of dehairing of hides and skins is the chemical process using lime and sodium sulphide. It is estimated that less than 2% of the beamhouses use enzymes for dehairing. The sulphide primarily acts by cleaving the disulfide links of the keratin molecules. This action is aided by calcium hydroxide (lime), which loosens the collagen structure through swelling and releases interfibrillar noncollagenous proteins. This process is the conventional hair-burn or pulping system.
Enzymatic dehairing methods are known as an environmentally-friendly alternative to the conventional chemical process. Examples of enzymatic dehairing are described in U.S. Pat. No. 3,840,433, U.S. Pat. No. 4,636,222, WO 94/06942, U.S. Pat. No. 5,834,299 and WO 2008/093353. The enzyme digests the basal cells of the hair bulb and the cells of the malphigian layer (the two innermost layers of the epidermis). This is followed by loosening of hair with an attack on the outermost sheath and subsequent breakdown of the inner root sheath and parts of the hair that are not fully keratinized. Enzymes used in dehairing are generally proteolytic which catalyzes the breakdown of proteins. Examples of proteases which have been used are more or less crude protease extracts of bacterial or fungal origin containing different peptidase activities, as well as more pure proteases such as elastase, subtilisins, trypsins, chymotrypsin, aspartic proteases, cysteine protease and metalloproteases. However, since hides and skins primarily are made up of collagens which are susceptible to degradation by protease, there is a risk of grain damage to the skin or hide when using proteases. Furthermore, proteases may not be able to remove the hair completely, leaving an undesired stubble and potentially an uneven color on the skins or hides.
Continuous efforts are needed to design an ideal enzyme for dehairing, which provides sufficient hair removal and minimum damage to the leather. Furthermore, the generation of a more environment-friendly beamhouse process is also desired.